This invention relates to a mask blank which is used in a lithography process on fabricating a semiconductor device or the like and a method of manufacturing the mask blank. In addition, this invention also relates to a method of removing a useless film partially left on a surface of a substrate, such as a semiconductor substrate, and a removing apparatus for carrying out the method. Although the term “useless” will be mainly used in the instant specification, the term “unnecessary” may be used instead of the term “useless”.
In a field of manufacturing a semiconductor device, mask blank, and the like, it is often required to partially remove a useless portion from a film deposited on a principal surface of a substrate.
For example, let consideration be made about coating a resist film or a spin on glass (SOG) film on the substrate by a spin coat method. In this event, the substrate that is substantially kept horizontal is rotated. Under the circumstances, a coating agent or liquid is dripped on the substrate and is spread on the principal surface by centrifugal force to form a coated film on the principal surface. Herein, it is to be noted that, when the substrate is rotated at a slow speed so that a uniform film is coated on a whole of the principal surface, the centrifugal force becomes weak at a peripheral portion of the substrate and the coating liquid is undesirably left at the peripheral portion. As a result, the coated film is thick at the peripheral portion to form a hump thereat. Such a hump at the peripheral portion gives rise to the following problems when a resist is used as the coating liquid.
In a manufacturing process of a mask blank, such as a photo-mask blank, each substrate with the resist film is subjected to various processes and is often grasped by a transfer mechanism and/or is entered in or out of a substrate cassette during the processes. In this event, the peripheral portion of the substrate is brought into contact with a chuck of the transfer mechanism and with a slot or recess of the cassette. Such contacts bring about peeling off the resist film at the peripheral portion. Peeled off resist acts as a dust source and adheres to the principal surface of the mask blank and, as a result, causes an undesired defect to occur on the mask blank.
Taking the above into account, processing is often adopted such that the resist film is previously removed from the peripheral portion after the resist film is coated by the spin coating method. Such processing is carried out by horizontally rotating the substrate with the resist film around a rotation axis, by supplying a solution liquid of dissolving the resist onto the peripheral portion, and by removing the resist film from the peripheral portion.
Such a method is exemplified in Japanese Patent Publication JP 58-19350 B2, namely, 19350/1983. The disclosed method is carried out by placing a hollow cover of a pyramidal configuration on a substrate surface and by supplying a solvent from an apex onto the peripheral portion of the substrate.
In the meantime, it is a recent trend that a mask blank, especially, a photo-mask blank becomes large in size, for example, from a substrate size of 5×5 (inches) to 6×6 (inches). As the substrate becomes large in size, its weight becomes heavy. This impertinently heightens a possibility of occurrence of dust due to contacts between the peripheral portion of the substrate and a mask case, when the substrate with the resist film is introduced into the mask case. In this viewpoint also, requirements have been directed to removing the resist film coated at the peripheral portion.
Moreover, it often happens that supplementary patterns, such as alignment marks, barcode patterns, quality assurance (QA) patterns, and the like are arranged on a mask substrate except real patterns. The supplementary patterns tend to be located at an area very close to a peripheral edge of the substrate with an expansion of an effective area for the real pattern.
Now, description will be made about shortcomings of the conventional method mentioned in the above.
(1) The resist film can be removed along four sides of the photo-mask blank only in a constant removed width that is comparatively wide. The resist film is removed until an area of forming the supplementary patterns, which renders the supplementary patterns defective.
(2) The photo-mask blank is processed by a photolithography technique into a reticle for delineating a fine pattern on a semiconductor wafer by the use of an exposure device. In this event, the reticle is fastened or chucked by a reticle chuck prepared in the exposure device. Herein, it is to be noted that the reticle chuck is different in structure from one another at every maker for the exposure device. At any rate, the reticle is held on a substrate table in each exposure device with the reticle adsorbed by the reticle chuck, so as to avoid displacement of the reticle. Specifically, the reticle is supported with three points adsorbed by the reticle chuck because a plane can be defined by three points. The three adsorbed points should not be displaced because accuracy of position in the exposure device is very important so as to form a fine pattern on a semiconductor wafer. However, the resist film can not be removed in consideration of a configuration of the reticle chuck and accuracy of the position is inevitably deteriorated in the exposure device.
(3) With a short wavelength of exposure light, recent requirements have been directed to a mask blank of a high quality having a small number of defects. Under the circumstances, a plurality of mask blanks (usually 5 in number) are accommodated or packaged into a mask case for shipment. In this event, each of the mask blanks should be identified in the mask case so as to distinguish them from one another. To this end, the mask case is usually given defect degree data of each mask blank packaged in the mask case. The defect degree data on each mask blank should be collated with reference to each position of the mask blanks packaged in the mask case. However, collation of the defect degree data becomes difficult once each mask blank is picked out of the mask case to be investigated and is placed into a different position of the mask case. In this case, defects of the mask blanks in the mask case should be measured again.
(4) An area of forming supplementary patterns, such as alignment marks, is usually different in specification at every mask manufacturer. In this event, the alignment marks can not be often formed when the same cover is used to remove the useless portions by partially dissolving the resist film. This is because using the same cover brings about undesirably removing the peripheral portions of the mask substrate to positions of forming the alignment marks.
In order to avoid such undesirable removal of the resist film, considerations may be made about designing and preparing covers that provide alignment marks different from one another at every mask manufacturer. However, this drives up costs and requires troublesome replacement of the covers and laborious maintenance.